1. Industrial Applicability
The present invention relates to a method of manufacturing a non-volatile semiconductor memory device for storing information by accumulation of an electric charge. More specifically, the present invention relates to a method of manufacturing a non-volatile semiconductor memory device in which a memory element is selectively activated by a field effect transistor, and to the non-volatile semiconductor memory device manufactured by accordance with this method.
2. Description of the Background Art
As a non-volatile semiconductor memory device having memory elements provided with floating gates and control gates, flash memory can be mentioned as an example. A variety of designs are available for flash memories, one of which is a device in which a memory element is selectively activated by a field effect transistor. A flash memory with such a configuration has been disclosed, for example, in Japanese Patent Application Laid-Open No. 6-275847/1994. In the following, a method of manufacturing the flash memory disclosed in Japanese Patent Application Laid-Open No. 6-275847/1994 is described with reference to FIGS. 44 to 52.
As shown in FIG. 44, on a principal surface of a semiconductor substrate 200, a silicon oxide layer 202 as a tunnel oxide layer is grown, and then a polysilicon layer 204 as a floating gate is formed. Part of the polysilicon layer 204 that is positioned over an access transistor formation region 232 is selectively etched as shown in FIG. 45, and remaining part of the polysilicon layer 204 positioned over a memory element formation region 234 is left. This remaining part of the polysilicon layer 204 is hereinafter referred to as a polysilicon layer 204a. As shown in FIG. 46, an ONO-layer 206 is formed on the polysilicon layer 204a, and a silicon oxide layer 208 as a gate oxide layer is formed over the access transistor formation region 232. Subsequently, a polysilicon layer 210 is formed on the ONO-layer 206 and the silicon oxide layer 208.
As shown in FIG. 47, a resist 212 is prepared on the polysilicon layer 210, which is then selectively etched by using the resist 212 as a mask, thereby forming a gate electrode 214 over the access transistor formation region 232 while leaving part of the polysilicon layer 210 that is positioned over the memory element formation region 234. The remaining part of the polysilicon layer 210 over the memory element formation region 234 is hereinafter referred to as a polysilicon layer 210a. This etching exposes the silicon oxide layer 208 on a principal surface 236 of the semiconductor substrate 200, in the area between the gate electrode 214 and a floating gate to be formed in a later step. Next, as shown in FIG. 48, the resist 212 is removed and a resist 216 is prepared over the memory element formation region 234 and the access transistor formation region 232. The resist 216 is patterned so that it provides a mask for forming a control gate.
Note that the resist 216 is patterned so that it covers the gate electrode 214, while at the same time its side surface 216a does not overlap the polysilicon layers 204a and 210a. The gate electrode 214 has to be covered because the gate electrode 214 is formed of a material identical to that of the control gate and the floating gate, i.e. polysilicon, and therefore has to be protected from being etched away during the etching step to form the control gate and the floating gate. The patterning is provided in such a way that the side surface 216a does not overlap the polysilicon layers 204a and 210a because, when the polysilicon layers 204a and 210a are etched later to form the control gate and the floating gate, unnecessary polysilicon layers 204a and 210a are left on the principal surface of the semiconductor substrate 200 if the side surface 216a overlaps the polysilicon layers 204a and 210a. Consequently, the resist 216 is patterned while maintaining the silicon oxide layer 208 exposed on a principal surface 236 of the semiconductor substrate 200, in the area between the gate electrode 214 and a floating gate to be formed in a later step.
The polysilicon layer 210a is selectively etched by using the resist 216 as a mask to form a control gate 218. The ONO-layer 206 is then selectively etched by using the resist 216 as a mask, as shown in FIG. 49. This etching removes the exposed portion of the silicon oxide layer 208 and exposes the principal surface 236 in the area between the gate electrode 214 and a floating gate to be formed in a later step.
As shown in FIG. 50, the polysilicon layer 204a is selectively etched by using the resist 216 as a mask, thereby forming a floating gate 220. Since the principal surface 236 is exposed, the principal surface 236 is also etched to unavoidably form a groove section 222 on the principal surface 236. Subsequently, an ion implantation is provided on the principal surface of the semiconductor substrate 200 using the resist 216 as a mask, thereby forming a source/drain region 224 in the memory element formation region 234 as well as an impurity region 226 electrically connected to the source/drain region 224 in the groove section 222.
A silicon oxide layer 228 is grown on the principal surface of the semiconductor substrate 200 as shown in FIG. 51, followed by the formation of a contact hole 238 on the silicon oxide layer 228 so that the source/drain region 224 is exposed. As shown in FIG. 52, an aluminum wiring layer 230 is then provided on the silicon oxide layer 228. The aluminum wiring layer 230 is also formed on the contact hole 238 and is electrically connected to the source/drain region 224. A memory element 242 is provided with the control gate 218, the floating gate 220, and the source/drain region 224, whereas an access transistor 244 is provided with the gate electrode 214 and the source/drain region 240.
Referring to FIG. 52, for selectively activating the memory element 242 with the access transistor 244, the source/drain region 240 of the access transistor 244 and the source/drain region 224 of the memory element 242 are electrically connected through the impurity region 226 formed within the groove section 222. Since the wiring region comprising the source/drain region 240, the impurity region 226, and the source/drain region 224 has an irregular shape due to the presence of the groove section 222, the diffusion resistance of the impurity region 226 significantly affects the diffusion. resistance of the wiring region. In the meantime, as described with reference to FIG. 50, the source/drain region 224 and the impurity region 226 are formed simultaneously by a single ion implantation. Since this implantation is performed under the conditions for depth and concentration of impurities required for forming the source/drain region 224, the depth and concentration of impurities at the impurity region 226 are not at the adequate levels required for the region. This leads to undesirable consequences where, for example, a high diffusion resistance at the impurity region 226 slows the speed of programming, erasing, and reading of the memory element 242.
The present invention has been made to eliminate the above-described problems with the prior art. Accordingly, an object of the present invention is to provide a non-volatile semiconductor memory device and a manufacturing method thereof, wherein at least one of either the source/drain region of the access transistor or source/drain region of the memory element can be formed with a required thickness and concentration of impurity, and also the diffusion resistance of the impurity region formed at the groove section can be lowered.
The present invention provides a manufacturing method of a non-volatile semiconductor memory device comprising: at least one memory element including: a semiconductor substrate having a principal surface comprising a first region and a second region; a floating gate formed on the first region; a control gate formed on the floating gate; a first source/drain region formed in the first region; and a second source/drain region formed in the first region apart from the first source/drain region, located by the floating gate and the control gate therebetween; and
at least one access gate transistor for selectively activating the memory element, the access gate transistor comprising: a gate electrode formed on the second region; a third source/drain region that is formed in the second region and is electrically connected to the second source/drain region; and a fourth source/drain region formed in the second region apart from the third source/drain region, located by the gate electrode therebetween,
wherein the manufacturing method comprising:
a step of forming a tunnel insulation layer on the first region;
a step of forming on the tunnel insulation layer a first conductive layer which becomes the floating gate;
a step of forming a dielectric layer on the first conductive layer;
a step of forming a gate insulation layer on the second region;
a step of forming a second conductive layer on the dielectric layer and the gate insulation layer;
a step of forming the control gate and the gate electrode by selectively etching the second conductive layer; and
a step of forming the floating gate by selectively etching the first conductive layer,
wherein, when the first conductive layer is selectively etched, the principal surface in the area between the floating gate and the gate electrode is also unavoidably etched to form a groove section, and
wherein the manufacturing method further comprising:
a step of performing a first ion implantation in the semiconductor substrate in a manner to cover the groove section so that a first impurity region is formed in the semiconductor substrate; and
a step of performing a second ion implantation in the semiconductor substrate in a manner to cover the groove section so that a second impurity region and at least one of the first, second, third, and fourth source/drain regions are formed in the semiconductor substrate, the second impurity region overlapping with the first impurity region at the groove section, the second source/drain region and the third source/drain region being electrically connected by the first impurity region and the second impurity region.
According to the method of manufacturing a non-volatile semiconductor memory device of the present invention, the first impurity region is formed on the principal surface by a first ion implantation provided over the area of the groove section, and subsequently the second ion implantation is performed over the area of the groove section, thereby forming on the principal surface the second impurity region which overlaps the first impurity region at the groove section and at the same time electrically connects the second source/drain to the third source/drain with the first impurity region. The impurity region at the groove section comprises the first and second impurity regions which overlap one other. Since the impurity region at the groove section is formed by a two-step doping of the first and second ion implantations, this helps lower the diffusion resistance. Consequently, the speed of programming, erasing, and reading of the memory element can be improved. Meanwhile, since the impurity regions at the groove section are formed separately by the first and second ion implantations, the second ion implantation can be performed under conditions that provide for the depth and concentration of impurities required for the source/drain to be formed by the ion implantation.
As an aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that the first impurity region is formed by performing the first ion implantation using the first resist and the second resist as masks, the first resist covering the first region in which the first source/drain is formed; and a side surface of the first resist being positioned on the control gate, the second resist covering the second region in which the fourth source/drain is formed; and a side surface of the second resist being positioned between the gate electrode and the groove section;
the first, third, and fourth source/drain regions, and the second impurity regions are formed by performing the second ion implantation in the semiconductor substrate using the control gate and the gate electrode as masks; and
the second source/drain region is formed by performing the first and the second ion implantations. Since the first, third, and fourth source/drain regions are formed by the second ion implantation, these regions can be provided with the depth and concentration of impurities required for the respective source/drain region. Moreover, since the side surface of the first resist is not positioned between the control gate and the groove section, it dispenses with the need to consider the mask alignment margin, which enables shortening the distance between the control gate and the groove section, thereby achieving a high cell density and a high degree of integration for the non-volatile semiconductor memory device.
As another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that the first impurity region is formed by performing the first ion implantation using a third resist and a forth resist as masks, the third resist covering the first region in which the first source/drain region is formed; and a side surface of the third resist being positioned between the control gate and the groove section, the fourth resist covering the second region in which the fourth source/drain region is formed; and a side surface of the forth resist being positioned between the gate electrode and the groove section; and
the first, second, third, and fourth source/drain regions, and the second impurity regions are formed by performing the second ion implantation in the semiconductor substrate using the control gate and the gate electrode as masks.
As yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that the first impurity region is formed by performing the first ion implantation using a fifth resist and a sixth resist as masks, the fifth resist covering the first region in which the first source/drain region is formed; and a side surface of the fifth resist being positioned on the control gate, the sixth resist covering the second region in which the fourth source/drain region is formed; and a side surface of the sixth resist being positioned on the gate electrode;
the first and fourth source/drain regions and the second impurity region are formed by performing the second ion implantation in the semiconductor substrate using the control gate and the gate electrode as masks; and
the second and third source/drain regions are formed by performing the first and second ion implantations. Since the first and fourth source/drain regions are formed with the second ion implantation, these regions can be provided with the depth and concentration of impurities required for the respective source/drain region. Moreover, since the side surface of the fifth resist is not positioned between the control gate and the groove section, this dispenses with the need to consider the mask alignment margin, which enables shortening the distance between the control gate and the groove section. Furthermore, since the side surface of the sixth resist is not positioned between the gate electrode and the groove section, there is no need to allow for the mask alignment margin, which enables shortening the distance between the gate electrode and the groove section. Consequently, the present embodiment enables achieving a higher cell density and a higher degree of integration for the nonvolatile semiconductor memory device than in the preferred embodiments of the present invention previously described.
As yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that the first impurity region is formed by performing the first ion implantation using a seventh resist and a eighth resist as masks, the seventh resist covering the first region in which the first source/drain region is formed; and a side surface of the seventh resist being positioned between the control gate and the groove section, the eighth resist covering the second region in which the fourth source/drain region is formed; and a side surface of the eighth resist being positioned on the gate electrode;
the first, second, and fourth source/drain regions and the second impurity region are formed by performing the second ion implantation in the semiconductor substrate using the control gate and the gate electrode as masks; and
the third source/drain region is formed by performing the first and second ion implantations. Since the first, second, and fourth source/drain regions are formed with the second ion implantation these regions can be provided with the depth and concentration of impurities required for the respective source/drain. Moreover, since the side surface of the eighth resist is not positioned between the gate electrode and the groove section, this dispenses with the need to consider the mask alignment margin, which enables shortening the distance between the gate electrode and the groove section. Consequently, as with other embodiments of the present invention previously described, the present embodiment enables achieving a high cell density and a high degree of integration for the non-volatile semiconductor memory device.
As yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that the first and second source/drain regions and the first impurity region are formed by performing the first ion implantation using the control gate and a ninth resist as masks, the ninth resist covering the second region in which the fourth source/drain region is formed; and a side surface of the ninth resist being positioned between the gate electrode and the groove section; and
the third and fourth source/drain regions and the second impurity region are formed by performing the second ion implantation using the gate electrode and a tenth resist as masks, the tenth resist covering the first region in which the first source/drain region is formed; and a side surface of the tenth resist being positioned between the control gate and the groove section. Since the first and second source/drain regions are formed by performing the first ion implantation and the third and fourth source/drain regions are formed by the second ion implantation, the first, second, third, and fourth source/drain regions can be provided with the depth and concentration of impurities required for the respective source/drain region.
Yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention may comprise: prior to the step of forming the tunnel insulation layer, a step of forming on the principal surface an element-isolating insulation layer that includes a side portion located in the first and second regions and isolates the memory element and the access gate transistor from other memory element and other access gate transistor,
wherein the step of forming the first impurity region includes the first ion implantation using as a mask an eleventh resist that covers the element-isolating insulation layer, the side surface of the eleventh resist being positioned outside the side portion of the element-isolating insulation layer.
When forming the first impurity region in a deep position, ions have to be implanted at a high energy level. However, since the element-isolating insulation layer alone cannot stop the ions piercing through the element-isolating insulation layer to reach the semiconductor substrate, the element-isolating insulation layer eventually loses its function of element isolation. Accordingly, a resist is formed on the element-isolating insulation layer, and the first ion implantation is performed using this resist as a mask to form the first impurity region. If the side surface of the resist is positioned inside the side portion of the element-isolating insulation layer at the time of the first ion implantation, the first impurity region can also be formed underneath the side portion of the element-isolating insulation layer to punch through other impurity regions. In the present embodiment, however, since the first impurity region is formed by performing the first ion implantation using the eleventh resist as a mask which covers the element-isolating insulation layer with the side surface of the resist being positioned outside the side portion of the element-isolating insulation layer, a distance is created between the side surface of the first impurity region and the side portion of the element-isolating insulation layer. Consequently, the first impurity region can be prevented from diffusing underneath the element-isolating insulation layer to punch through other impurity regions, even if the first impurity region is formed in a deep position. In this configuration, the distance from the side surface of the eleventh resist to the side portion of the element-isolating insulation layer is preferably 0.1 xcexcm or more and 0.3 xcexcm or less. Moreover, it is preferable to apply the present invention when the width of the element-isolating insulation layer is 2 to 3 xcexcm or less.
As yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that serial steps for forming the second conductive layer up to forming the floating gate comprising:
a step of forming a twelfth resist on the second conductive layer, after a step of forming the second conductive layer;
a step of selectively etching the second conductive layer using the twelfth resist as a mask to leave a portion of the second conductive layer on the first region, and to form the gate electrode;
a step of forming a thirteenth resist on the second conductive layer in the first region and to cover the gate electrode;
a step of selectively etching the second conductive layer using the thirteenth resist as a mask to form the control gate; and
a step of selectively etching the first conductive layer using the thirteenth resist as a mask to form the floating gate.
As yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that serial steps for forming the second conductive layer up to forming the floating gate comprising:
a step of forming a first insulation layer on the second conductive layer, after a step of forming the second conductive layer;
a step of forming a fourteenth resist on the first insulation layer; and
a step of selectively etching the first insulation layer and the second conductive layer using the fourteenth resist as a mask to simultaneously form the control gate and the gate electrode,
wherein the first insulation layer remains on the control gate and the gate electrode, and
wherein the serial steps further include:
a step of forming a fifteenth resist so as to cover the gate electrode; and
a step of selectively etching the first conductive layer using the first insulation layer on the control gate and the fifteenth resist as masks to form the floating gate.
When the control gate and the gate electrode are formed separately, there is a need to consider the margin for aligning the mask for forming the control gate and the mask for forming the gate electrode. This necessitates a certain distance allowing the mask alignment to be placed between the control gate and the gate electrode. In the present embodiment, however, since the control gate and the gate electrode are simultaneously formed, there is no need to consider the margin for aligning the masks for forming the control gate and the gate electrode. Consequently, the distance between the control gate and the gate electrode can be reduced to contribute to microminiaturization of the non-volatile semiconductor memory device.
As yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that serial steps for forming the second conductive layer up to forming the floating gate comprising:
a step of forming a second insulation layer on the second conductive layer, after a step of forming the second conductive layer;
a step of forming a sixteenth resist on the second insulation layer;
a step of selectively etching the second insulation layer using the sixteenth resist as a mask; and
a step of selectively etching the second conductive layer using the second insulation layer as a mask to simultaneously form the control gate and the gate electrode,
wherein the second insulation layer remains on the control gate and the gate electrode, and
wherein the serial steps further include:
a step of forming a seventeenth resist so as to cover the gate electrode; and
a step of selectively etching the first conductive layer using the second insulation layer on the control gate and the seventeenth resist as masks to form the floating gate. For yet unknown reasons, etching can be performed more accurately by masking with an insulation layer, rather than with a resist. Since the present embodiment forms the control gate using the second insulation layer as a mask, it is able to form a control gate of a more accurate shape than a method using a resist as a mask.
As yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, non-volatile semiconductor memory device preferably comprise a plurality of the memory elements and a plurality of the access gate transistors, each one of the access gate transistors selectively activating only one memory element.
As yet another aspect of the method of manufacturing a non-volatile semiconductor memory device of the present invention, it is preferable that the groove section is formed to a depth of between 100 to 300 nm;
the step for forming the first impurity region comprises:
the first ion implantation performed initially with phosphorus under conditions of an energy of 40 to 120 KeV and a dosage of 1E14/cm2 to 6E15/cm2, then with any one of phosphorus and arsenic under conditions of an energy of 30 to 80 KeV and a dosage of 1E15/cm2 to 6E15/cm2; and
a step of thermally treating the implanted ion under N2 or N2/O2 atmosphere, at a temperature between 900 and 950xc2x0 C., and for a duration of 30 to 180 minutes to form the first impurity region having a thickness of 200 to 600 nm and an impurity concentration of 1E18/cm3 to 1E21/cm3; and
the step for forming the second impurity region comprises:
the second ion implantation performed initially with phosphorus under conditions of an energy of 40 to 120 KeV and a dosage of 5E12/cm2 to 5E14/cm2, then with any one of phosphorus and arsenic under conditions of an energy of 30 to 80 KeV and a dosage of 1E15/cm2 to 6E15/cm2; and
a step of forming the second impurity region having a thickness of 100 to 400 nm and an impurity concentration of 1E17/cm3 to 1E21/cm3.
The steps for forming the second impurity region in this embodiment comprise the second ion implantation performed initially with phosphorus and under conditions of energy of 40 to 120 KeV and a dosage of 5E12 to 5E14/cm2, then with either phosphorus or arsenic under conditions of 30 to 80 KeV and 1E15 to 6E15/cm2, to form the second impurity region having a depth of 100 to 400 nm and an impurity concentration of 1E17 to 1E21/cm3. Note that the thermal treatment for the diffused ions will be provided concurrently with a thermal treating step later in the manufacturing process.
The present invention farther provides a non-volatile semiconductor memory device storing information by an accumulation of electric charge, comprising:
a semiconductor substrate having a principal surface comprising a first region and a second region;
at least one memory element including: a floating gate formed in the first region; a control gate formed on the floating gate; a first source/drain region formed in the first region; and a second source/drain region formed in the first region apart from the first source/drain region, located by the floating gate and the control gate therebetween;
at least one access gate transistor for selectively activating the memory element, the access gate transistor comprising: a gate electrode formed in the second region; a third source/drain region formed in the second region; and a fourth source/drain region formed in the second region apart from the third source/drain region, located by the gate electrode therebetween,
wherein a groove section is unavoidably formed in the semiconductor substrate between the floating gate and the gate electrode, and
wherein the non-volatile semiconductor memory device further comprises an impurity region formed in the semiconductor substrate so as to cover the groove section, the impurity region electrically connecting the second source/drain region and the third source/drain region, and having a higher impurity concentration than the first and the fourth source/drain regions.
As an aspect of the non-volatile semiconductor memory device of the present invention, it is preferable that the impurity concentration in the impurity region is at least one-and-a-half times but not more than twice the impurity concentration in the first and fourth source/drain regions.
As another aspect of the non-volatile semiconductor memory device of the present invention, it is preferable that the impurity concentration in the impurity region is the same as the impurity concentration in the second source/drain region and higher than that in the first, third, and fourth source/drain regions.
As yet another aspect of the non-volatile semiconductor memory device of the present invention, it is preferable that the impurity concentration in the impurity region is higher than the impurity concentration in the first, second, third, and fourth source/drain regions.
As yet another aspect of the non-volatile semiconductor memory device of the present invention, it is preferable that the impurity concentration in the impurity region is the same as the impurity concentration in the second and third source/drain regions and higher than that in the first and fourth source/drain regions.
As yet another aspect of the non-volatile semiconductor memory device of the present invention, it is preferable that the impurity concentration in the impurity region is the same as the impurity concentration in the third source/drain region and higher than that in the first, second, and fourth source/drain regions.
Yet another aspect of the non-volatile semiconductor memory device of the present invention may comprise: an element-isolating insulation layer that is formed on the principal surface and includes a side portion located in the first and second regions and that isolates the memory element and the access gate transistor from other memory element and access gate transistor;
a first impurity region formed in the semiconductor substrate, and being apart from the element-isolating insulation layer; and
a second impurity region formed in the semiconductor substrate so that a portion thereof overlaps the first impurity region, having a smaller thickness than the first impurity region, and being in contact with the element-isolating insulation layer. In this aspect, the distance between the element-isolating insulation layer and the first impurity region is preferably between 0.1 and 0.3 xcexcm.
Note that when the word xe2x80x9conxe2x80x9d is used to explain the positional relationship between two layers of the device in the present application, there may exist other layers between these two layers.